Since mask blanks are processed into masks (transfer masks), typically photomasks, for use in the fabrication of microelectronic devices, they must be free of defects such as particles and pin holes on their surface. With respect to the size of defects, the pattern miniaturization technology requires that the number of defects with a size of greater than 0.1 μm be zero.
Such submicron defects are generated by numerous factors. To elucidate the cause-and-effect relationships between all factors and defects, separate experiments are necessary for every factor. Overall experimentation is a quite difficult task. One factor of generating defects in a mask blank is assigned to, in the case of sputter deposition of a film on a substrate, those film portions deposited on chamfers and side surfaces of the mask blank. Since the process of manufacturing a mask blank includes the step of handling a mask blank substrate or a mask blank, contact marks by the handling tool are necessarily left on the substrate. If the point of contact during the handling step is positioned on a portion of the front or back surface of the substrate that is subject to exposure on use of the mask, this positioning is undesirable because the contact mark becomes a factor of causing defects such as flaws and particles. It is generally recommended that the point of contact during the handling step is positioned on the side surface, chamfer, front surface-chamfer boundary, and back surface-chamfer boundary.
On use of the mask obtained from the mask blank, the chamfer and side surface of the substrate are not the portion of the mask that contributes to pattern transfer. Thus the chamfer and side surface of the substrate are not considered a subject of defect inspection. Since the conventional defect inspection unit makes access to the film formed on the front surface of the substrate, high-precision inspection is not performed on the chamfer and side surface of the substrate even though flaws or particles are on these sites. Thus it remains unconfirmed how a film is deposited at these sites.
If flaws or particles are present on the substrate or film of the mask blank, a film deposited on the substrate or film (in the latter case, a film disposed remote from the substrate) becomes fragile or peelable at the flaw or particle-covering site due to weak bonding force. If a film is deposited on the side surface, chamfer, front surface-chamfer boundary, or back surface-chamfer boundary of the substrate, the film tends to peel off on these sites. If spalled film fragments migrate onto the front surface of the mask blank, they become particle defects. This causes to increase defects on the mask blank. Peeling of the film at these sites may be promoted by the washing and transporting steps during mask blank manufacture and the step of processing the mask blank into a mask.